Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images which show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions which emanate from the body and are captured by a scintillation crystal, with which the photons interact to produce flashes of light or “events.” Events are detected by an array of photodetectors, such as photomultiplier tubes, and their spatial locations or positions are calculated and stored. In this way, an image of the organ or tissue under study is created from detection of the distribution of the radioisotopes in the body.
One particular nuclear medicine imaging technique is known as Positron Emission Tomography, or PET. PET is used to produce images for diagnosing the biochemistry or physiology of a specific organ, tumor or other metabolically active site. Measurement of the tissue concentration of a positron emitting radionuclide is based on coincidence detection of the two gamma photons arising from positron annihilation. When a positron is annihilated by an electron, two 511 keV gamma photons are simultaneously produced and travel in approximately opposite directions. Gamma photons produced by an annihilation event can be detected by a pair of oppositely disposed radiation detectors capable of producing a signal in response to the interaction of the gamma photons with a scintillation crystal. Annihilation events are typically identified by a time coincidence between the detection of the two 511 keV gamma photons in the two oppositely disposed detectors, i.e., the gamma photon emissions are detected virtually simultaneously by each detector. When two oppositely disposed gamma photons each strike an oppositely disposed detector to produce a time coincidence event, they also identify a line of response, or LOR, along which the annihilation event has occurred.
An example of a PET method and apparatus is described in U.S. Pat. No. 6,858,847, which patent is incorporated herein by reference in its entirety. After being sorted into parallel projections, the LORs defined by the coincidence events are used to reconstruct a three-dimensional distribution of the positron-emitting radionuclide within the patient. PET is particularly useful in obtaining images that reveal bioprocesses, e.g. the functioning of bodily organs such as the heart, brain, lungs, etc. and bodily tissues and structures such as the circulatory system.
The PET apparatus includes a detector section constructed of blocks, each forming an array of scintillation crystals, a data acquisition section, and an event processing section. FIG. 1. shows an example of a data acquisition section applicable to the present invention. Analog front-end circuitry 101 (such as Application-Specific Integrated Circuits (ASICs)) connects to external detectors such as Photomultiplier Tubes (PMTs) or Avalanche Photodiodes (APDs) (not shown), which in turn are coupled to scintillation crystal blocks (not shown). Energy data in the form of analog signals are outputted from the analog front-end circuitry to Analog-to-Digital Converters (ADCs) 103, which convert the analog signals to digital data samples. The digital energy data samples outputted from the ADCs are passed to a Field Programmable Gate Array (FPGA) 105. For the Siemens Inveon® PET system, sixteen data samples are accumulated for each event initiated by a Constant Fraction Discriminator (CFD) trigger from the analog front-end circuitry.
In addition to the digital energy data samples, a digital time stamp with a 312 picosecond resolution is outputted from the analog front-end circuitry, which also is passed to the FPGA 105 and used in subsequent event processing. Once received by the FPGA, the digital data samples may be processed as needed for the particular application. The data samples may also be outputted from the FPGA for analysis and processing on a host machine in addition to subsequent processing within the FPGA.
FIG. 2 illustrates details of the FPGA 105. Various algorithms may be applied to the accumulated digital data samples within the FPGA for determining the optimum data representation of the event represented by the data samples. The selected algorithm is executed on the accumulated data samples by an Event Representation module 201. The resultant data representation of an event is then used by X,Y Calculation module 203 to calculate an X,Y spatial coordinate. The coordinate is subsequently used to address a Crystal Look-up Table (CLT) 209 that is implemented in a Flash memory device external to the FPGA. A crystal value is then outputted from the CLT 209 dependent upon the X,Y coordinate that addressed the CLT.
This crystal value is then fed back into the FPGA 105 and used to address an Energy Qualification and Time Correction look-up table (ELT) 205 implemented within the FPGA. ELT 205 stores upper and lower energy values and a time correction value for each crystal. This allows energy qualification and time correction to be applied to each individual crystal. Once a crystal event has been qualified as to energy and corrected as to timing based on the identified crystal in which the event occurred, the corrected data form a “Singles” event that is then placed into a FIFO buffer 207 for transmission over an I/O channel for subsequent processing, such as coincidence determination.
Prior to the present invention, crystal identification mapping was based on square or rectangular maps utilizing the entire X,Y area of the scintillation crystal array, as shown in FIG. 3. This technique provided a binary decision as to the position of the event based on the crystal data stored in the CLT, such that each event is mapped to a particular crystal and is assumed to have occurred at the centroid of the crystal. Because all events incident within the entire detector field of view are captured, the sensitivity of the detector is maximized. However, events detected as occurring on the boundary between two crystals could be mispositioned, depending on the peak/valley ratio of the given detector. This results in a statistical degradation of the resolution of the final image.
Circular or “island” mapping is also known in the art, wherein circular regions are formed around the centroids of the scintillation crystals. The use of smaller regions provides increased confidence that the detected event actually occurred in the specified crystal; however sensitivity is reduced because events that are detected as occurring in areas outside the circular regions are discarded. Further, the radius of the regions needs to be determined in advance, or unnecessary loss of sensitivity will result, as reduction of the radius of the circular regions at some point dramatically reduces the overall image quality because of the extreme loss of statistical data.